1. FIELD OF THE INVENTION
This invention relates to a bleaching composition which possesses improved storage stability.
2. DESCRIPTION OF THE PRIOR ART
Inorganic peroxides that release hydrogen peroxide in aqueous solutions, such as sodium perborate and sodium percarbonate, are widely utilized as bleaching agents for commercial use and household use.
These inorganic peroxides exhibit a sufficient bleaching effect when the bleaching treatment is conducted at a high temperature such as 80.degree. C. or higher, but at lower temperatures their bleaching effects are insufficient. There are known various activating agents for improving their bleaching effect at lower temperatures. These known activating agents are roughly divided into three groups, namely N-acyl compounds, esters and organic acid anhydrides. Such activating agents react with the inorganic peroxides in an aqueous solution to form an organic peracid. It is considered that the bleaching effect is enhanced by the thus-obtained increase of the oxidation potential. If such an inorganic peroxide and an activating agent therefore are mixed to form a bleaching composition, the composition can be conveniently used soon after it is prepared. However, if they are simply mixed and then the bleaching composition is stored, the storage stability of this composition is drastically reduced as compared with the storage stability of the inorganic peroxide alone. Thus bleaching compositions composed of a blend of an inorganic peroxide and an activating agent therefore do not possess adequate storage stability, i.e. the effective oxygen concentration in bleaching solutions made therefrom is greatly reduced if the compositions are stored for an appreciable period prior to use.
Some attempts have previously been made on special combinations of inorganic peroxides and activating agents to overcome the foregoing defects and to improve the storage stability of the bleaching composition. For instance, U.S. Pat. No. 3,639,248 teaches absorbing an inorganic acid anhydride, employed as an activating agent, on a granular inert substance, and British Pat. No. 906,358 teaches coating an N-acyl compound, employed as an activating agent, with a water-soluble film-forming substance such as polyvinyl alcohol and carboxymethyl cellulose. Further, British Pat. No. 864,798 proposes increasing the grain size of an ester used as the activating agent. We tried these known method and found that each of these proposals is not fully effective. Especially, the effect attained by the method of U.S. Pat. No. 3,639,248, which teaches that an acid anhydride is a typical example of the activating agent, is very inferior in storage stability. When the grain size of the activating agent is increased, as taught in British Pat. No. 864,798, if an ester of very low water-solubility such as glucose pentacetate is used, the bleaching effect is reduced at low temperature.
It is, therefore, a primary obect of this invention to provide an improved bleaching composition in which the activities of the inorganic peroxide and the activating agent are maintained at a high level and to provide a bleaching composition having an improved stability.